1. Field of the Invention
This invention relates to an automatic focus system most suitable for cameras.
2. Description of Related Art
In cameras with automatic focus systems, the camera drives the lens and performs the operations of focussing on the primary subject when the photographer points the lens at the primary subject and depresses the release button halfway so that the camera is in a range measurement state. However, if the photographer accidentally places the primary subject outside the field of vision of range measurement, the camera again drives the lens and focuses instead on whatever non-primary subject the photographer has accidentally placed within the field of vision of range measurement.
When the primary subject and non-primary subjects are adjacent to one another, it is conceivable that the action of the camera focussing on the non-primary subject can be completed in a relatively short time and the photographer can easily return the focus state of the camera to focussing on the primary subject. However, when the non-primary subject is far from the primary subject, a longer time is required for the photographer to return the primary subject to the camera's field of vision for range measurement and to return the camera once again to a state wherein it is focussed on the primary subject, thereby missing photographic opportunities.
Among the conventional technologies designed to solve such problems, there are systems which have close-range limits and far-range limits for the lens and which have a limit for driving the lens to far ranges or to short ranges above a certain level; systems which have software which prevents the camera from refocussing as long as the photographer does not repeat the range measurement operation when the camera has once accomplished focus recognition; and systems such as set forth in Japanese Patent Disclosure Sho 60-53913 which allow the photographer to arbitrarily set the range for the focus operation performed by the camera.
However, because with these conventional technologies the camera's focus operation is controlled or prevented without regard to the nature of the subject, the problem arises that it impossible to chase a subject beyond an established limit if the subject is moving.